We request funds for the purchase of an FEI Tecnai G2 Spirit BioTwin transmission electron microscope (TEM) with embedded CCD camera. This will serve the biomedical research needs of eleven NIH-funded investigators at the University of Massachusetts Medical School (UMMS) and Boston University School of Medicine. The microscope will be housed in the Core EM Facility of UMMS, and maintained and monitored by the Facility Manager, ensuring efficient use by investigators. It will replace a 23-year old, second-hand Philips CM12, which is based on outdated technology, lacks key capabilities required by users, and for which maintenance is likely to be discontinued or become difficult in the near future. The new instrument would immediately solve these problems. The projects that will use the Tecnai include fundamental studies of autophagic and apoptotic cell death, mechanisms of synapse formation, the regulation of contraction in smooth and striated muscle, the role of MAP kinase signaling pathways in neurodegeneration, the function of centrosomes in normal and diseased cells, the regulation of G-protein-coupled receptors, the structure and function of spliceosomes, and the structure and function of cilia and flagella in normal and diseased states. These projects require TEM as an essential tool for investigating cellular and molecular architecture at high resolution and as a means of detecting and localizing specific cell molecules by immuno-gold labeling. The images obtained will provide essential insights into mechanisms of cell development, function, infection and disease. In addition to the users in this application, there are numerous other investigators at UMMS who use TEM on a more occasional basis and who would also benefit from modernization of our TEM technology. The instrument proposed represents the state-of-the-art for a conventional TEM, and has many features that would greatly enhance TEM productivity at UMMS. These include a modern, user-friendly computer interface, motorized stage controls with 180o tilt capability, the ability to store and recall grid locations and alignment and imaging parameters (especially useful in several studies requiring serial sectioning), and full integration of an embedded CCD camera, making it possible to record images rapidly and with full documentation. All of the projects that will use the microscope aim to provide fundamental insights into cellular structure and function. Most have a direct bearing on the understanding of human disease. Access to this state-of-the-art instrument will play a key role in advancing our knowledge in these biomedically important areas.